Measure battery condition (safety critical unit)?

That is the usual way of doing it. It may not work very well for a device that's going to be stored for a long time with the battery disconnected; you need to choose the right battery chemistry for the application. (Sounds like you have to solve some of the same problems as an automatic [parachute] activation device; take a look at how the pros do it in that application)

"critical" voltage should be well above the brownout voltage for the circuit, so you can simply turn on the load and see how far Vbat dips.

Have you considered adding a capacitor (maybe an ultracap)? If it's a short term draw then the cap may allow you to operate with a much lower peak power draw from the battery.

Robert

If your system is really safety critical you should consider a complete re-think so that you do not need an active event to achieve a safe state. Obviously 'safety critical' covers a multitude of possibilities but if there is a significant risk of causing death or serious injury I would not advise you to rely on a complex loop involving a battery test and a decision by your software.

Michael Kellett

For the application I described, it is SOP for the unit to do a self- test when the user powers it up (before they step in the plane) and report any problems at that time. It doesn't do the test immediately before the critical event; it does the test when you indicate you will soon be relying on it.